Quantitative Energy Transfer in Organic Nanoparticles Based on Small-Molecule Ionic Isolation Lattices for UV Light Harvesting

Fluorescent nanoparticles based on organic dyes are promising materials for bioimaging applications. Recently, ultrabright fluorescent nanoparticles with orange emission were obtained by hierarchical coassembly of a cationic rhodamine dye with cyanostar anion-receptor to produce small-molecule ionic isolation lattices (SMILES). The cyanostar anion-complexes provides spatial and electronic isolation of the rhodamine dye prohibiting aggregation quenching. Cyanostar also constitutes a UV excitation antenna system to boost the brightness of the rhodamine SMILES nanoparticles due to a large molar absorption coefficient in the UV region and efficient energy transfer to the dye. To further study the UV light harvesting process, we compared the rhodamine SMILES nanoparticles to green emissive cyanine-based SMILES nanoparticles, different in spectral overlap between cyanostar and the dye molecules. The energy transfer efficiency is increased from 80% in rhodamine SMILES to 100% in cyanine SMILES NPs due to increased spectral overlap. The energy transfer process was studied in detail by using femtosecond (fs) transient absorption (TA) spectroscopy, yielding energy transfer time-constants of around 0.4 and 1.3 ps for the cyanine- and rhodamine-based SMILES NPs, respectively. This result correlates well with the spectral overlap integrals and accounts for increased energy transfer and UV light harvesting efficiency. This insight into the UV light energy harvesting processes in the supramolecular SMILES materials will aid future design of ultrabright functional nanomaterials.